1. Field of the Invention
The present invention relates to refractories used for submerged nozzles and the like for use in continuous casting of steel. In particular, the present invention relates to a zirconia-carbon-containing refractory having high corrosion resistance and high thermal shock resistance.
2. Description of the Related Art
A submerged nozzle for use in continuous casting of steel is used to transfer molten steel from a tundish to a mold. The submerged nozzle is used to prevent molten steel from coming into contact with air to inhibit oxidation of molten steel and used to charge molten steel into the mold while the flow of molten steel is adjusted. This results in the prevention of contamination of steel with slag layer floating on the surface of molten steel and nonmetallic inclusions in molten steel, thereby improving the quality of steel and ensuring the stability of operation. In general, a molten glass layer, referred to as a “mold powder layer”, is present on the surface of molten steel in the mold. The molten glass layer contains CaO, SiO2, Na2O, K2O, Al2O3, CaF2, C, and the like and is thus highly erosive to Al2O3, SiO2, C, and the like constituting the submerged nozzle, so that operation over long periods of time reduces the corrosion resistance of the submerged nozzle. Thus, a portion of the submerged nozzle coming into contact with the mold powder is often composed of a zirconia material having corrosion resistance against molten glass. To ensure thermal shock resistance, zirconia-carbon (ZrO2—C) material is generally used as a powder line material.
Various improvements in the corrosion resistance of the powder line material have been achieved because the corrosion resistance directly affects the lifetime of the nozzle. In general, it is known that an increase in zirconia content of the material improves the corrosion resistance. Meanwhile, a larger zirconia content increases the thermal expansion coefficient and the elastic modulus of the ZrO2—C material, disadvantageously causing breaks in use and hindering the operation. To improve the thermal shock resistance, the graphite content needs to be increased. As described above, however, an increase in graphite content reduces the corrosion resistance; hence, it is important to strike a balance between the zirconia content and the graphite content. In general, from the viewpoint of stably using the submerged nozzle, the upper limit of the amount of zirconia aggregates incorporated is about 90% by mass.
For a submerged nozzle composed of several types of materials such as an alumina-graphite material or an alumina-silica-graphite material, a partially stabilized aggregate or a completely stabilized aggregate raw material containing 3% to 10% by mass of CaO, MgO, Y2O3, or the like exhibiting relatively linear thermal expansion characteristics is generally applied from the viewpoint of thermal structural stability in receiving molten steel. The upper limit of the proportion of a ZrO2 component in a ZrO2—C material used for the powder line portion is about 86% by mass because of the incorporation of bonding carbon that bonds aggregates together. To use a high corrosion-resistant powder line portion that has a low incidence of breaking and can contribute to stable operation, the proportion of the ZrO2 component is generally 82% by mass or less.
For example, Japanese Unexamined Patent Application Publication No. 11-302073 discloses a zirconia graphite refractory having excellent corrosion resistance and containing 70% to 95% by mass of a zirconia material and 5% to 30% by mass of graphite, in which zirconia particles each having a diameter of 45 μm or less account for 70% or more of the total amount of the zirconia particles.
Japanese Unexamined Patent Application Publication No. 8-1293 discloses a technique in which a portion of a submerged nozzle, used for continuous casting, coming into contact with a molten mold powder is composed of a zirconia-graphite material containing 50% to 90% by mass of a CaO-stabilized zirconia raw material having a silica content of 0.30% by mass or less, 0% to 30% by mass of a baddeleyite raw material (provided that the total amount of the CaO-stabilized zirconia raw material and the baddeleyite raw material is 60% to 91% by mass), and 10% to 35% by mass of a graphite raw material.
The zirconia-graphite refractory and the zirconia-graphite material described in the foregoing Patent Documents, however, do not sufficiently have both thermal shock resistance and corrosion resistance in a high production operation nowadays.
A zirconia-graphite material which does not break by thermal shock in operation and has better corrosion resistance than those of the materials described above is thus required.
Hitherto, at a ZrO2 component content of about 80% by mass or less, a larger ZrO2 component content results in improvement in corrosion resistance against the powder. A ZrO2 component content exceeding about 80% by mass, however, is liable to lead to a reduction in corrosion resistance. Thus, the upper limit of the ZrO2 component content is about 83% by mass.